The invention described herein was made by employees of the United States Government. The invention may be manufactured and used by or for the governmental purposes without the payment of royalties thereon or therefor.
The invention is directed to a method and apparatus for, determining two-dimensional absolute position through optical encoding.
A two-dimensional optical encoder measures a two-dimensional position of an object by optically detecting marks on a scale attached to the object which moves with the object. Ordinarily, when two axes of encoding are required in an application, two linear encoders, disposed orthogonal to each other, are deployed, one along each axis. However, measurement errors can occur if the travel axes are not straight, flat, or orthogonal. Measurement error can also occur if the linear encoders are not aligned properly to their directions of motion. In this case, measured travel is always less than actual travel. In the case where an axis does not travel straight, such that with the intended motion there is also movement orthogonal to the intended motion, the encoder along the orthogonal axis will not sense that movement, resulting in error in position measurement. Therefore, the measurements can be of limited accuracy due to lack of orthogonality of mechanical alignment of axes, non-straight travels in two orthogonal axes, and mechanically coupled motions from two orthogonal axes.
If the encoders are of the incremental type each of the encoders simply measures translation by counting the number of marks that move past the encoder""s optical detector. In a common form of such an encoder per axis, a fixed scale and a moving scale, which have identical transparent markings on opaque backgrounds, are interposed between a light source and the detector. The relative locations of the transparent markings determine the amount of light which is allowed to be transmitted through each marking, e.g., full transmission, xc2xd or xc2xc transmission, or none at all. Of course, such an encoder can measure only relative displacement with respect to a reference positionxe2x80x94not absolute position.
In a conventional absolute encoder, each position is given not simply by just one mark, but by a unique code pattern of marks which identifies the absolute position. A change in position is sensed by detecting a change in the code bits which make up the code pattern.
In a conventional absolute encoder such as the one just described, sensitivity is limited to the size of the smallest code bit which can be recorded, which is in turn limited by physical optics to about the wavelength of the light used to record and detect the code patterns. Thus, the best sensitivity available from such an absolute encoder is somewhat less than 1 micrometer (xcexcm) of translation. Also, such an encoder is limited in the amount of travel that it can accommodate. For instance, such an encoder which uses 12-bit code patterns can encode up to 212=4,096 positions. With a sensitivity of just under 1 micrometer (xcexcm), the maximum travel which can be detected is around 4,000 micrometer (xcexcm), or four millimeters. Moreover, because the code bits themselves are detected, damage to the scale can result in dead spots in which derived position information is anomalous.
The encoder of the present invention has a scale having a pattern formed thereon, the pattern being predetermined to indicate an absolute location on the scale, means for illuminating the scale, means for forming an image of the pattern; and detector means for outputting signals derived from the portion of the image of the pattern which lies within a field of view of the detector means, the field of view defining an image reference coordinate system, and analyzing means, receiving the signals from the detector means, for determining the absolute location of the object. There are two types of scale patterns presented in this invention: Cartesian grid type and starfield type, although other types such as skew and polar are also practical.
The invention is further directed to a method of determining a two-dimensional absolute position of an object for both grid type and starfield type patterns. In general, the method comprises: (a) providing a scale having a pattern formed thereon, the pattern being predetermined to indicate an absolute two-dimensional location on the scale; (b) relating motion between the scale and the object; (c) forming an image of the pattern which lies within a field of view of the detector means; and (d) determining a two-dimensional absolute location on the scale in accordance with the image.
The encoder and method according to the present invention use mature technologies such as microlithography, optical projection, CCD array image detection, and simple computational image processing. However, the specific use of a combination of such mature technologies according to the present invention allows results, such as those stated above, which were not possible through the use of heretofore known encoders.